Metal stamping tools often use nitrogen gas springs. Such springs, however, exhibit a linear force-to-travel distance relationship. There are situations, where a non-linear or discontinuous force-to-travel distance relationship (or force jump) would be advantageous in metal stamping operations. For example, certain hard-to-form metals, such as high strength steels and aluminum alloys, have a tendency to spring back after stamping to an unacceptable shape.
One solution to such unacceptable metal spring-back is to apply a significantly different force to the metal at different points in the stamping operation. For example, causing the stamping tool to apply a significantly greater force to the metal via the springs near the end of a stamping cycle acts as a post stretching force, which has been found to minimize or eliminate undesirable spring back as described, e.g., in “SHAPESET: A process to Reduce Sidewall Curl Springback in High-Strength Steel Rails: published in the Journal of Applied Metalworking, in January, 1984.
Attempts to accomplish changes in the force-to-travel relationship of the stamping tool springs have involved complicated systems that adjust the amount of fluid (gas or liquid) within the compression chamber of the spring. U.S. Pat. No. 7,257,460, incorporated herein by reference, is an example of such complicated systems, which require hydraulic pumping units, manifold piping systems, hydraulic cooling systems, additional power systems, etc. Such complex systems are costly to implement and operate.
In addition to the complexity and costs of such additional systems, any such fluid adjustment takes time, which negates the possibility of an instantaneous discontinuity or force jump, and requires an interruption or otherwise increases the stamping cycle time. Increasing the stamping cycle time reduces the number of parts that a metal stamping tool is able to stamp during a shift. Increased stamping cycle time increases both operational costs and capital costs of additional stamping tools that must be purchased to meet demand. All of these increased costs must be amortized over the number of parts each tool is able to stamp. Thus, even small increases in stamping cycle time often dramatically increase the costs per stamped part. As a result, it is desirable to provide a metal stamping tool with springs that are capable of instantly changing its force-to-travel relationship during a stamping cycle.